Bending the curve of biodiversity loss requires the business and financial sectors to disclose and reduce their biodiversity impacts and help fund nature recovery. This has sparked interest in developing generalizable, standardized measurements of biodiversity—essentially a ‘unit of nature’. We examine how such units are defined in the rapidly growing voluntary biodiversity credits market and present a framework exploring how biodiversity is quantified, how delivery of positive outcomes is detected and attributed to the investment and how the number of credits issued is adjusted to account for uncertainties. We demonstrate that there are deep uncertainties throughout the process and question if the benefits of biodiversity credits, and other efforts to abstract nature to a single unit, outweigh the harms. Credits can only be positive for biodiversity if they are used with unprecedentedly strict regulation that ensures businesses mostly avoid negative impacts and if they are purchased to quantify positive contributions rather than as direct offsets. While there may be a role for markets in attracting conservation funding, they will only ever be part of the solution, especially for the many aspects of nature that cannot be reduced to a unit.